This invention relates to variable focal length lenses or so-called zoom lenses for use with photographic cameras or television cameras and, more particularly, to a zoom lens objective having focusing control suited for photography at wide angle. Still more particularly, it relates to improvements of a zoom lens objective of the type disclosed in applicants' copending application Ser. No. 746,203 filed Dec. 1, 1976.
This type of zoom lens objective consists of three distinct sections, a zooming section having a plurality of movable lens groups by which the focal length of the entire optical system can be continuously varied throughout a range while maintaining constant the position of the image plane for an object at any distance, a focusing section positioned in front (on the object side) of the zooming section and having a function of providing a constant object point (image point of the object being photographed) with respect to the zooming section regardless of change in the object distance, and a fixed lens group (relay lens) positioned in the rear (on the image side) of the zooming section, the objective being well corrected for the various aberrations throughout the zooming range as well as throughout the focusing range.
In recent zoom objectives, the zooming range must be extended toward shorter focal lengths with simultaneous achievement of wider image angles and also must have a focusing control element capable of focusing down to shorter object distances. For these purposes, the part of the objective to be movable for focusing is required not only to provide an increased lens aperture but also to be of a relatively complex structure, or otherwise it would be made impossible to secure the desired wide angle of view at the minimum focal length region and to achieve good state of correction of the aberrations not only throughout the zooming range but also throughout the focusing range from the infinitely focused position to close up positions. This complexity makes the proportion of the weight of the focusing control lens group quite large relative to the rest of the complete objective with the corresponding increase in the weight of the focusing mechanism as compared with the prior art.
In order to minimize the size and weight of the complete objective including its operating mechanism to easily manageable proportions, it has been the prior art practice, instead of making the front lens group bodily movable for focusing, to divide such front lens group into two parts. One such part is movable for focusing and another part which remains stationary during focusing and which is especially designed to facilitate correction of the various aberrations, as, for example, disclosed in British Pat. No. 975,160. There the front lens group is divided into a negative part and a positive part, only the negative part being axially movable. U.S. Pat. No. 3,682,534 calls for the front lens group consist of two negative sub-groups only the rear sub-group being axial movable. U.S. Pat. No. 3,598,476 discloses that the front lens group is divided into a negative front part, a positive intermediate part with and a negative rear part only the intermediate part being axially movable toward the front with decrease in the object distance. The zoom objective adapted for macro-range photography mention may be those disclosed in Japanese patent publication Nos. Sho 47- 42174, Sho 49-7752 and Sho 49-7753.